Coordinate measuring machines are used for dimensional inspection of workpieces such as machine parts. A workpiece is secured to a table, and a measuring probe is mounted to a movable ram. In order to measure the coordinates of a point on the workpiece, the probe is brought into contact with the point. X, y and z measuring scales that indicate the coordinates of the probe in the measurement volume of the machine are sensed and recorded. State-of-the-art coordinate measuring machines have refinements such as high resolution measuring systems, electrical contact probes, motor drives, computer control drives and computer acquisition and processing of coordinate data.
The probe used to contact the points on the workpiece is typically in the form of spherical ball at the end of a shaft attached to the movable ram. The coordinates sensed by the system are the coordinates of the center point of the probe. However, the actual point of contact between the probe and the workpiece is a point on the surface of the spherical ball. Thus, the coordinate values sensed by the system differ from the actual coordinates of the point on the workpiece by the radius of the spherical ball, commonly known as "probe offset." Furthermore, since the probe may approach the workpiece from any direction, the direction between the actual contact point and the probe center is variable from measurement to measurement.
In addition to measuring coordinates of points on the workpiece, coordinate measuring systems typically can measure various geometric shapes such as planes, straight lines, circles, cylinders, cones and spheres. A plane is defined by a vector normal to the plane and the coordinate of the intersection, between the plane and the normal vector. Circles, cylinders and spheres are defined by the coordinates of a center point or center axis and the magnitude of the radius.
In compensating for probe offset, the magnitude of the probe offset is known and fixed. However, the direction of the probe offset is not known to the measuring system and is not fixed. For example, the probe can approach a planar surface from either direction and from different angles. A curved surface can be an outside diameter, such as a cylindrical surface, or an inside diameter, such as a hole in a part. Thus, it has been difficult to automatically compensate measured coordinates for probe offset.
In a prior art system, an operator was required, prior to each set of measurements, to manually enter certain parameters, such as inside diameter, outside diameter or direction of approach to a plane, into the coordinate measuring machine computer. The system then determined the required direction of probe offset compensation. While this technique is in theory reliable, errors were often made by the operator, requiring repeated measurements or providing erroneous data. In addition, the speed of the system was reduced by the necessity to manually enter such information.
It is a general object of the present invention to provide an improved coordinate measuring machine.
It is another object of the present invention to provide a method for automatically compensating for probe offset in a coordinate measuring machine.
It is a further object of the present invention to provide a highly accurate method for automatic probe offset compensation.
It is yet another object of the present invention to improve the speed and accuracy of coordinate measuring machines.
It is a further object of the present invention to provide coordinate measuring machines with simple operating procedures.